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Interpretive Summary: Application of classical insecticides has introduced severe problems in agricultural sustainability. The concept of biological control of insects is a potentially powerful alternative to classical insecticides. Biological control is based on the idea that direct application of insect-specific pathogens and parasites can reduce pest insect populations and the economic damage due to pest insects. The problem, however, is the efficiency of these organisms in biological control programs is limited by insect immune defense reactions to challenge. One approach to improving the efficiency of biocontrol agents would be to somehow disable insect immune reactions to viral, bacterial, fungal and parasitic infections. With this goal, we are investigating how insect immune reactions to infection are signaled. In this paper we report on identification of a new protein that may be responsible for stimulating insect defenses to infection. This new research will be directly useful to scientists who are working to improve the efficacy of biological control methods. The ensuing improved biological control methods will benefit a wide range of agricultural producers by supporting the long-term sustainability of agriculture.

Technical Abstract: The biogenic amine serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter in vertebrates and invertebrates. It acts in regulation and modulation of many physiological and behavioral processes through G protein-coupled receptors. Insects express five 5-HT receptor subtypes that share high similarity with mammalian 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B and 5-HT7 receptors. We isolated a cDNA (Pr5-HT8) from larval Pieris rapae, which shares relatively low similarity to the known 5-HT receptor classes. After heterologous expression in HEK-293 cells, Pr5-HT8 mediated increased [Ca**2+]i in response to low concentrations (<10 nM) of 5-HT. The receptor did not affect [cAMP]i even at high concentrations (>10 uM) of 5-HT. Dopamine, octopamine and tyramine did not influence receptor signaling. Pr5-HT8 was also activated by various 5-HT receptor agonists including 5-methoxytryptamin, (+ or -)-8-Hydroxy-2-(dipropylamino) tetralin and 5-carboxamidotryptamine. Methiothepin, a nonselective 5-HT receptor antagonist, activated Pr5-HT8. WAY 10635, a 5-HT1A antagonist, but not SB-269970, SB-216641 or RS- 127445, inhibited 5-HT-induced [Ca**2+]i increases. We infer that Pr5-HT8 represents the first recognized member of a novel 5-HT receptor class with a unique pharmacological profile. We found orthologs of Pr5-HT8 in some insect pests and vectors such as beetles and mosquitoes, but not in the genomes of honeybee or parasitoid wasps. This is likely to be an invertebrate-specific receptor because there were no similar receptors in mammals.